1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method to be used in order to transfer a mask pattern onto a substrate through a projection optical system in the lithography step for producing, for example, semiconductor elements, liquid crystal display elements, plasma display elements, or thin film magnetic heads. In particular, the present invention relates to an exposure apparatus and an exposure method capable of controlling the distribution of illumination generated by an illumination system automatically or precisely.
2. Description of the Related Art
In order to respond to the improvement in the degree of integration and the degree of fineness of the semiconductor device, the exposure apparatus, which is in charge of the lithography step (representatively including the resist application step, the exposure step, and the resist development step) for producing the semiconductor device, is required to further enhance, for example, the resolving power and the transfer faithfulness. In order to enhance the resolving power and the transfer faithfulness as described above, it is necessary that the wavelength of the exposure light beam as the exposure beam is shortened, the projection optical system having a large numerical aperture is used, and the exposure amount is controlled highly accurately in order to expose, with a proper exposure amount, the photoresist applied on the wafer as the substrate. In order to extract the image formation characteristic of the projection optical system to the limit so that the exposure amount is controlled for the photoresist highly accurately, it is necessary to optimize the illumination optical system so as to enhance the illumination characteristic of the illumination optical system for illuminating the reticle as the mask with the exposure light beam as far as possible.
The adjustment to optimize the illumination optical system of the exposure apparatus has been hitherto performed in accordance with the following steps.
(a) An operator measures the illumination characteristic (for example, uneven illuminance) of an adjustment objective of the illumination optical system.
(b) The state (for example, position or angle of inclination) of a predetermined optical member is adjusted by using a driving unit corresponding to the illumination characteristic on the basis of the obtained result of the measurement. The driving amount concerning this process is set so that the illumination characteristic is improved as far as possible by correcting the optical design value on the basis of the experience of the operator.
(c) After the adjustment, the remaining amount of the illumination characteristic is measured again. If the remaining amount exceeds an allowable range, the adjustment is performed again by the aid of the driving unit.
(d) After the completion of the adjustment, the final state (optimum state) of the optical member is stored.
The adjustment steps as described above are repeated for every illumination characteristic of the adjustment objective for each of a plurality of illumination conditions to store the optimum state of the corresponding optical member. When the illumination condition is switched, the corresponding optical member is set to be in the optimum state respectively.
As described above, the adjustment for optimizing the illumination optical system of the conventional exposure apparatus has been performed by the operator, for example, when the exposure apparatus is assembled and adjusted and when the maintenance is performed.
However, when the operator performs the adjustment, an inconvenience arises such that a long period of time is required to perform the adjustment. Further, it is necessary to adjust the illumination optical system for each of the plurality of illumination conditions. Therefore, the overall adjustment time is fairly prolonged. The time required for the optimization is also affected by the degree of skill of the operator. Therefore, there has been also such a fear that the adjustment time is further prolonged depending on the operator.
When the states of a plurality of optical members in the illumination optical system are required to be adjusted, it is necessary to consider, for example, the mutual influence caused by the adjustment as well. Therefore, the adjustment steps have been extremely complicated.
As described above, the adjustment for the conventional illumination optical system has required the complicated steps which take a long period of time. Therefore, for example, it has been difficult to perform such an operation that the allowable level of a predetermined illumination characteristic is changed depending on, for example, the required accuracy for the device to be produced. Further, for example, the uneven illuminance of the illumination characteristic is changed in a time-dependent manner, for example, due to the cloudiness of the optical member in the illumination optical system and the deterioration of the saltpeter material in some cases. However, in such a case, it has been difficult for the conventional adjustment method to make quick response.
The uneven illuminance is principally divided into the uneven illuminance which is axially symmetrical with respect to the optical axis (centro-symmetrical unevenness), i.e., the quadratic function-like unevenness, and the inclination unevenness in which the illuminance is gradually increased or decreased in the area across the optical axis, i.e., the linear function-like unevenness. It is necessary that the uneven illuminance as described above is corrected highly accurately in the orthogonal two directions in the case of the full field exposure type exposure apparatus such as the stepper. On the other hand, in the case of the scanning exposure type exposure apparatus such as those based on the step-and-scan system, the uneven illuminance in the scanning direction is averaged by the scanning exposure operation to such an extent that little problem occurs. Therefore, it is required to especially correct the uneven illuminance in the non-scanning direction perpendicular to the scanning direction highly accurately.
The uneven illuminance has been hitherto corrected in ordinary cases by driving a group of predetermined lenses in the illumination optical system in the optical axis direction, or by driving the group of lenses so that the tilt angle about the two axes is changed. In general, the situation of the uneven illuminance differs depending on the illumination condition. Especially, in the case of the centro-symmetrical unevenness, the degree of concaveness/convexness of the illuminance is changed corresponding to the change of the position through which the light flux passes in the lens group depending on the numerical aperture of the exposure light beam (illumination light beam). Therefore, for example, as for the lens group as the adjustment objective, the optimum position is previously stored for each of the illumination conditions, and the lens group is driven to the optimum position every time when the illumination condition is changed.
A phenomenon is known, in which the cloudy substance adheres to the surface of the optical element when the exposure light beam in the ultraviolet region reacts with a minute amount of organic matter contained in the gas existing around the optical element. Usually, the gas, from which the organic matter or the like is removed, for example, through a chemical filter, is supplied to the surroundings of the respective lenses of the illumination optical system and the projection optical system. However, when the exposure apparatus is used for a long period of time, then a slight amount of remaining organic matter gradually increases the cloudiness of the lens, and the centro-symmetrical unevenness, in which the illuminance is lowered especially at the central portion, is sometimes advanced in a time-dependent manner. In such a case, the operator has adjusted the position of the corresponding lens group again, depending on the degree of advance of the centro-symmetrical unevenness. Further, for example, when the centro-symmetrical unevenness is extremely advanced during the process of the use of the exposure apparatus for several years, the lens group as the adjustment objective itself is exchanged with a lens group having a strong effect of correction in some cases.
As described above, the uneven illuminance of the conventional exposure apparatus has been corrected by controlling the tilt angle or the position of the predetermined lens group including the lens having a certain curvature (refractive power) in the optical system, or by exchanging the lens group with another lens group.
However, when it is intended to correct the uneven illuminance by driving the lens having a certain curvature other than the flat plane, the uniformity of the coherence factor ("sgr" value) of the illumination light beam is occasionally deteriorated in the illumination area on the reticle as the mask, and in the exposure area on the wafer as the substrate to be exposed. When the uniformity of the "sgr" value is deteriorated in the exposure area as described above, an inconvenience arises such that the line width uniformity, which is the original object of the suppression of the uneven illuminance, is lowered. On the other hand, if the correction amount of the uneven illuminance is set so that the deterioration of the uniformity of the "sgr" value is suppressed, a limit appears in the line width control accuracy.
Especially, in recent years, the design rule (standard line width) of the semiconductor device becomes finer and finer year by year. In order to further improve the line width control accuracy, it is demanded to develop an exposure method which makes it possible to improve the uniformity of the exposure amount distribution without deteriorating the uniformity of the "sgr" value.
Taking the foregoing viewpoints into consideration, a first object of the present invention is to provide an exposure apparatus which makes it possible to correctly adjust an illumination optical system for a short period of time.
Further, a second object of the present invention is to provide an exposure apparatus which makes it possible to substantially automatically adjust an illumination optical system which is capable of making switch to a plurality of illumination conditions.
Further, a third object of the present invention is to provide a method for efficiently using such an exposure apparatus and a method for producing a highly accurate device based on the use of such an exposure apparatus.
A fourth object of the present invention is to provide an exposure method which makes it possible to improve the uniformity of the exposure amount distribution without substantially deteriorating the uniformity of the coherence factor of an exposure light beam.
A fifth object of the present invention is to provide an exposure apparatus which makes it possible to carry out the exposure method provided in accordance with the fourth object.
A sixth object of the present invention is to provide a method for producing a device, which makes it possible to produce the device with a high line width control accuracy by using the exposure method according to the present invention.
According to a first aspect of the present invention, there is provided an exposure apparatus for exposing a second object with an exposure light beam via a first object, the exposure apparatus comprising: an illumination system which is provided with an optical member and which illuminates the first object with the exposure light beam; an illumination condition-switching system which is arranged in the illumination system and which switches an illumination condition of the first object with the exposure light beam; and
an adjusting system which adjusts a state of the optical member in the illumination system in order to control an illumination characteristic of the illumination system depending on the switched illumination condition.
According to the present invention as described above, when the illumination condition is switched with the illumination condition-switching system, the state of the optical member (for example, position in the optical axis direction, position in the direction perpendicular to the optical axis, and tilt angle) is adjusted by the aid of the adjusting system depending on the illumination condition after the switching operation. Accordingly, the predetermined illumination characteristic of the illumination system can be substantially automatically controlled to be in a desired state for a plurality of illumination conditions respectively. The adjusting system may include a driving system which moves the optical member, and a control system which controls the driving system depending on the switched illumination condition.
In this arrangement, an example of the predetermined illumination characteristic of the evaluation object is at least one of uneven illuminance of the exposure light beam and a collapse amount of a telecentric property (telecentricity) of the exposure light beam. Both of them are extremely important characteristics to obtain a high resolution on the second object. Further, it is desirable that the illumination characteristic of the evaluation objective includes an inclination component and a concave/convex component of the uneven illuminance of the exposure light beam, and inclination components (two-dimensional vector amounts) and a magnification component of the collapse amount of the telecentric property of the exposure light. The five components of the illumination characteristic can be easily controlled substantially singly by mutually independently driving a plurality of optical members in the illumination system. Therefore, it is especially easy to effect the automatization.
It is desirable that the exposure apparatus further comprises a characteristic-measuring system which measures the illumination characteristic of the illumination system, wherein the control system determines and stores a relationship between a driving amount of the driving system and an amount of change of the illumination characteristic on the basis of a result of the measurement performed by the characteristic-measuring system. When the illumination characteristic is changed in a time-dependent manner, the illumination characteristic can be quickly restored to a desired state, for example, by periodically measuring the illumination characteristic with the characteristic-measuring system, by updating the previously stored relationship by means of calculation (simulation), or by simultaneously using both of the foregoing means (i.e., by updating the relationship by means of calculation during the periodic measurement of the illumination characteristic) so that the optical member is driven on the basis of the obtained result.
According to a second aspect of the present invention, there is provided an exposure apparatus for exposing a second object with an exposure light beam via a first object, the exposure apparatus comprising:
an illumination system which is provided with an optical member and which illuminates the first object with the exposure light beam;
a characteristic-measuring system which measures an illumination characteristic of the illumination system; and
an adjusting system which adjusts a state of the optical member in accordance with a result of the measurement performed by the characteristic-measuring system.
According to the exposure apparatus as described above, the illumination system can be correctly adjusted for a short period of time by driving the adjusting system on the basis of the result of the measurement performed by the characteristic-measuring system provided with, for example, a spatial image-measuring system.
In the present invention as described above, when the illumination system includes an optical integrator (uniformizer or homogenizer) and a first optical system and a second optical system which introduce the exposure light beam passed through the optical integrator into an irradiation plane of the first object or a plane conjugate therewith, the following illumination characteristics can be controlled substantially mutually independently by performing the adjustment for the states of the foregoing optical members respectively as follows:
(a1) adjustment for the position of the optical integrator in the optical axis direction: magnification component of the collapse amount of the telecentric property of the exposure light beam;
(b1) adjustment for the position of the first optical system in the optical axis direction: concave/convex component of the uneven illuminance;
(c1) adjustment for the two-dimensional position of the second optical system in the direction perpendicular to the optical axis: inclination component of the collapse amount of the telecentric property (two-dimensional vector amount); and
(d1) adjustment for the tilt angle of the second optical system: inclination component of the uneven illuminance in the tilting direction. It is desirable that the tilting angle corresponds to the non-scanning direction perpendicular to the scanning direction in the case of the exposure apparatus based on the scanning exposure system, because of the following reason. That is, the uneven illuminance is averaged owing to the integral effect in the scanning direction, while it is desirable to make the correction with the tilt, because no averaging effect is generated in the non-scanning direction.
In the present invention described above, it is desirable that the illumination system further includes an optical element which sets an illuminance distribution of the exposure light beam to a local area for modified illumination, a beam-shaping optical system which introduces the exposure light beam from an exposure light source into the optical element, a light-collecting optical system which introduces the exposure light beam from the optical element, and the optical integrator which uniformizes the illuminance distribution of the exposure light beam from the light-collecting optical system, wherein the adjusting system adjusts the state of the light-collecting optical system or the beam-shaping optical system.
In this arrangement, for example, the beam-shaping optical system is adjusted so that the magnitude of the illuminance of the exposure light beam and magnitude of the dispersion of the illuminance distribution of the exposure light beam are balanced. Thus, it is possible to decrease the uneven illuminance on condition that the loss of the exposure light beam is minimized.
According to a third aspect of the present invention, there is provided an exposure apparatus for exposing a second object with an exposure light beam via a first object, the exposure apparatus comprising:
an illumination system which illuminates the first object with the exposure light beam;
a characteristic-measuring system which measures an illumination characteristic of the illumination system; and
a control system which independently determines a magnification component and an inclination component of a collapse amount of a telecentric property of the exposure light beam from the illumination characteristic measured by the characteristic-measuring system. It is easy to perform the adjustment substantially mutually independently by dividing the collapse amount of the telecentric property into the inclination component and the magnification component as described above. The control system may independently determine a concave/convex component and an inclination component of uneven illuminance of the exposure light beam afforded by the illumination system, from the illumination characteristic measured by the characteristic-measuring system.
According to a fourth aspect of the present invention, there is provided an exposure method for exposing a second object with an exposure light beam from an illumination system via a first object, the exposure method comprising the steps of:
illuminating the first object with the exposure light beam;
measuring an illumination characteristic of the illumination system;
independently determining a magnification component and an inclination component of a collapse amount of a telecentric property of the exposure light beam from the measured illumination characteristic;
adjusting the illumination system on the basis of the determined magnification and the inclination components of the collapse amount of the telecentric property; and
exposing the second object with the exposure light beam from the adjusted illumination system passing through the first object. According to this exposure method, the telecentric property of the illumination system can be adjusted easily for a short period of time. Thus, it is possible to improve the throughput. This method may further comprise the step of independently determining a concave/convex component and an inclination component of uneven illuminance of the exposure light beam afforded by the illumination system from the measured illumination characteristic.
According to a fifth aspect of the present invention, there is provided a method for adjusting an exposure apparatus provided with an illumination system for illuminating a first object with an exposure light beam, for exposing a second object with the exposure light beam via the first object, the method comprising the steps of:
setting a predetermined optical member in the illumination system in a plurality of states to measure an illumination characteristic of the illumination system in each state;
determining a relationship between an amount of change of the state of the optical member and an amount of change of the illumination characteristic on the basis of a result of the measurement of the illumination characteristic; and
adjusting the state of the optical member in order to control the illumination characteristic on the basis of the determined relationship. According to this adjusting method, it is possible to efficiently adjust the illumination characteristic by previously determining the relationship between the driving amount of the optical member and the amount of change of the illumination characteristic. The method may further comprise the step of storing the determined relationship.
According to a sixth aspect of the present invention, there is provided an exposure method for exposing a second object with an exposure light beam via a first object, the exposure method comprising the steps of:
radiating the exposure light beam onto the first object; and
controlling a transmittance distribution of the exposure light beam in a planer area traversing an optical axis of the exposure light beam in the vicinity of an exposure plane of the second object or in the vicinity of a plane conjugate with the exposure plane.
According to the exposure method as described above, the uneven illuminance is adjusted not by driving an optical element having a predetermined curvature other than the flat plane. The transmittance distribution of the planer area is controlled (especially controlled two-dimensionally) so that the uneven illuminance is corrected. Accordingly, the illuminance distribution can be controlled without substantially deteriorating the uniformity of the coherence factor of the exposure light beam in the exposure area of the second object. Therefore, the illuminance distribution is controlled so that the unevenness of the cumulative exposure amount on the second object is corrected, and thus it is possible to improve the uniformity of the exposure amount distribution so that the line width uniformity may be reliably improved. Further, the transmittance distribution is not fixed, it can be controlled variably two-dimensionally. Therefore, when any cloudiness or the like appears on the optical element such as those of the illumination system, and the illuminance distribution on the second object is changed in a time-dependent manner, then the transmittance distribution is controlled so that the change is offset. Thus, it is possible to always maintain high uniformity of the exposure amount distribution.
In this process, for example, the transmittance distribution for the exposure light beam is controlled to give a concentric distribution about the center of the optical axis so that the unevenness of the exposure amount distribution for the second object is corrected. When the concentric distribution is given as described above, it is possible to appropriately correct the uneven illuminance (centro-symmetrical unevenness) which is axially symmetrical with respect to the optical axis. Further, the centro-symmetrical unevenness can be corrected substantially continuously within a predetermined range by rotating the transmittance distribution.
In another example, the transmittance distribution for the exposure light beam is controlled to give a predetermined distribution in a first direction so that the unevenness of the exposure amount distribution for the second object is corrected. That is, the transmittance distribution is controlled to give a one-dimensional predetermined distribution. The direction of the one-dimensional distribution is variable. When the one-dimensional transmittance distribution is used as described above, it is possible to correct the unevenness of the cumulative exposure amount in the non-scanning direction (direction perpendicular to the scanning direction) when the scanning exposure is performed. That is, when the predetermined distribution is a distribution which changes one-dimensionally symmetrically about the center of the optical axis, the centro-symmetrical unevenness (quadratic function-like unevenness) can be corrected substantially continuously within a predetermined range after the scanning exposure. When the predetermined distribution is a distribution in which the transmittance is gradually increased or decreased as the position is separated farther from the optical axis, the inclination unevenness (linear function-like unevenness) can be corrected substantially continuously within a predetermined range after the scanning exposure.
The transmittance distribution for the exposure light beam may be further controlled in a direction intersecting the direction of the predetermined distribution with the same distribution as the predetermined distribution or with another distribution. When a plurality of one-dimensional transmittance distributions are combined, it is possible to correct the two-dimensional uneven illuminance such as the inclination unevenness and the centro-symmetrical unevenness in the stationary state. The present invention can be also applied to the exposure apparatus of the full filed exposure type such as the stepper. It is also possible to omit the mechanism for driving a large lens group to be used to correct the uneven illuminance.
In the scanning exposure system in which the first object and the second object are synchronously moved in a scanning direction when the second object is exposed, it is desirable that the transmittance distribution for the exposure light beam is controlled so that exposure amount distribution (exposure amount distribution in the non-scanning direction), which is obtained by adding up an exposure amount of the exposure light beam for the second object in the scanning direction, is uniformized. In this process, the uneven illuminance in the scanning direction is averaged by the scanning exposure. Therefore, the cumulative exposure amount distribution is uniformized on the entire surface of the second object by uniformizing the exposure amount distribution in the non-scanning direction perpendicular to the scanning direction. Thus, it is possible to obtain a high line width control accuracy.
According to a seventh aspect of the present invention, there is provided an exposure apparatus for exposing a second object with an exposure light beam via a first object, the exposure apparatus comprising:
an illumination system which illuminates the first object with the exposure light beam; and
at least one filter which is arranged in the vicinity of an exposure plane of the second object or in the vicinity of a plane conjugate with the exposure plane on an optical path for the exposure light beam up to the second object and which has a predetermined transmittance distribution with respect to the exposure light beam.
According to the present invention as described above, the transmittance distribution can be controlled substantially continuously, for example, by mechanically rotating the filter, or by electrically rotating the transmittance distribution of the filter. Therefore, it is possible to carry out the exposure method according to the sixth aspect of the present invention. In this arrangement, the apparatus may further comprise a driving unit which controls an angle of rotation of the filter itself or which makes control to electrically rotate the transmittance distribution of the filter. Accordingly, it is possible to automatically correct the unevenness of the exposure amount.
In this arrangement, the illumination system may include one stage of optical integrator or a plurality of stages of optical integrators for uniformizing an illuminance distribution of the exposure light beam, and a field diaphragm for defining an illumination area on the first object of the exposure light beam from the optical integrator; wherein the filter may be arranged on a plane in the vicinity of the field diaphragm or on a plane in the vicinity of an irradiation plane of the first object. Accordingly, the filter can be easily arranged.
The filter may be composed of two sheets of a first filter and a second filter which have the same transmittance distribution in a one-dimensional direction symmetrically with respect to an optical axis respectively, and the two sheets of the filters may be rotated in mutually opposite phases. Accordingly, the centro-symmetrical unevenness can be corrected continuously by means of the simple control. When the present invention is applied to an exposure apparatus based on the scanning exposure system, it is desirable that the exposure apparatus further comprises a stage system which synchronously moves the first object and the second object in a scanning direction; an exposure amount distribution-measuring unit which measures a distribution of added-up value in the scanning direction of an exposure amount of the exposure light beam on the second object; and a control unit which controls an angle of rotation of the filter by the aid of the driving unit in accordance with the exposure amount distribution measured by the distribution-measuring unit. In this arrangement, the unevenness of the exposure amount can be corrected highly accurately by measuring the distribution of the added-up value in the non-scanning direction with the distribution-measuring unit, and rotating the filter so that the distribution is uniform.
According to an eighth aspect of the present invention, there is provided an exposure apparatus for illuminating a first object with an exposure light beam and exposing a second object with the exposure light beam via the first object, the exposure apparatus comprising:
an illumination optical system which is capable of illuminating the first object under a plurality of illumination conditions respectively and in which an illuminance distribution of the exposure light beam has an identical tendency for the plurality of illumination conditions respectively; and
an optical member which is arranged on an optical path for the exposure light beam up to the second object and which adjusts the illuminance distribution.
According to the exposure apparatus of the present invention as described above, when the illumination condition is changed, for example, from the ordinary illumination to the modified illumination or the small "sgr" value illumination, the uniformity of the exposure amount distribution can be improved by offsetting the amount of change of the uneven illuminance by means of the optical member, for example, when the degree of the centro-symmetrical unevenness or the inclination unevenness is slightly changed.
In this arrangement, it is desirable that the illumination optical system is adjusted to have uneven illuminance in which the illuminance distribution is substantially symmetrical with respect to an optical axis of the illumination optical system. Accordingly, it is easy to correct the uneven illuminance. For example, the optical member includes at least one optical fiber which is arranged separately from an exposure plane of the second object or a conjugate plane thereof and which has a predetermined transmittance distribution with respect to the exposure light beam.
According to an ninth aspect of the invention, an exposure method for illuminating a first object with an exposure light beam and exposing a second object with the exposure light beam via the first object is provided. The exposure method comprises:
changing an illumination condition for the first object depending on a pattern to be transferred onto the second object;
adjusting an inclination component and a centro-symmetrical component of uneven illuminance or uneven exposure amount in an irradiation area of the exposure light beam, respectively; and
adjusting the centro-symmetrical component without adjusting the inclination component during a predetermined period after the adjustment of the uneven illuminance or uneven exposure amount.
According to an tenth aspect of the invention, an exposure method for irradiating a first object with an exposure light beam via an illumination optical system and exposing a second object with the exposure light beam via the first object is provided. The exposure method comprises:
detecting the exposure light beam on a predetermined plane on which the second object is arranged to measure an illumination characteristic including at least one of a distribution of exposure amount or illuminance in an irradiation area of the exposure light beam and a telecentricity of the illumination optical system;
moving at least one optical element of the illumination optical system on the basis of the measured illumination characteristic;
updating the measured illumination characteristic by means of calculation until the illumination characteristic is measured next time; and
moving the at least one optical element on the basis of the updated illumination characteristic.
A method for producing a device according to the present invention comprises the step of transferring a device pattern onto a substrate by using the exposure method according to any one of the aspects of the present invention. According to the present invention, it is possible to mass-produce a highly functional device with a high line width control accuracy.